1. Field of the Invention
The present invention relates to a silicon controlled rectifier (SCR) and an electrostatic discharge (ESD) structure and, more particularly, to a positive strike SCR, a negative strike SCR, and a bidirectional ESD structure that utilizes both the positive strike SCR and the negative strike SCR.
2. Description of the Related Art
A silicon-controlled rectifier (SCR) is a device that provides an open circuit between a first node and a second node when the voltage across the first and second nodes is positive and less than a trigger voltage. When the voltage across the first and second nodes rises to be equal to or greater than the trigger voltage, the SCR provides a low-resistance current path between the first and second nodes. Further, once the low-resistance current path has been provided, the SCR maintains the current path as long as the voltage across the first and second nodes is greater than a holding voltage, which is lower than the trigger voltage.
As a result of these characteristics, SCRs have been used to provide electrostatic discharge (ESD) protection. For example, the first node of an ESD SCR can be connected to a to-be-protected node, while the second node of the ESD SCR can be connected to a ground node. The ESD SCR operates within an ESD protection window that has a maximum voltage that is less than the destructive breakdown level of the to-be-protected node, and a minimum voltage that is greater than the holding voltage. The trigger voltage of the ESD SCR is then set to a value that is equal to or less than the maximum voltage of the window.
Thus, when the voltage across the to-be-protected node and the ground node is less than the trigger voltage, the ESD SCR provides an open circuit between the to-be-protected node and the ground node. However, when the to-be-protected node receives a voltage spike that equals or exceeds the trigger voltage, such as when an ungrounded human-body contact occurs, the ESD SCR provides a low-resistance current path from the to-be-protected node to the ground node. In addition, once the ESD event has passed and the current through the ESD SCR falls below a holding current of the ESD SCR, the ESD SCR again provides an open circuit between the to-be-protected node and the ground node.